The present invention relates to the field of hydraulics. More particularly, this invention relates to an electronic bore pressure optimization mechanism for altering the cylinder block piston bore pressure profile. The bore pressure profile has a direct impact on the noise, vibration, efficiency, and forces required to position the swashplate in a hydrostatic unit such as pump or motor. In general, the mechanism can be used to control any system variable, including but not limited to noise, vibration, flow ripple, pressure ripple, efficiency and/or the force and energy levels required to position the swashplate in axial piston pumps and motors. The mechanism is particularly useful in applications where operator xe2x80x9cfeelxe2x80x9d is important, allowing the operator to feel feedback from the vehicle but at reduced force levels. The mechanism is also useful in applications where system noise or sound level is important, allowing the reduction of noise in environments where sound levels must be regulated. The mechanism provides a dynamic or variable method of affecting or tuning net swashplate moments, sound, vibration, and/or efficiency.
xe2x80x9cFeelxe2x80x9d could be associated with almost any variable that can be sensed, including but not limited to: noise, control forces (power level), and flow ripple. Flow ripple is a well known and common phenomenon in multiple piston hydrostatic units. For instance, in an axial piston hydrostatic unit, the total average flow produced or consumed by a hydrostatic pump or motor is the sum over time of the flows produced or consumed by the individual pistons as they reciprocate when the cylinder block rotates. But the pistons are spaced apart along a pitch circle and are therefore phased in time such that the flow varies somewhat during each rotation of the cylinder block. The flow ripple comprises these variations in flow or deviations in amplitude from the average flow of fluid produced or consumed by the hydrostatic unit.
Hydrostatic transmissions have been used in skid steer loaders for a number of years now. In the early days of hydrostatically propelled skid steer loaders, the machines were relatively small and therefore the operator could manually control the position of the swashplate through mechanical linkage with minimal force and fatigue. The operator could also directly feel a feedback force from the swashplate. The energy or power to control the swashplate came solely from the operator. As the machines have become larger in recent years, the power and force levels have become too large for the operator to tolerate without tiring when operating the machine for an extended period of time.
Servo-controlled transmissions were developed to overcome the operator fatigue problem, but the operators then felt xe2x80x9cdisconnectedxe2x80x9d from the machine when attempting to control its displacement or swashplate position. The servo control devices require additional power and suffer reduced response capability, especially when response is needed most such as when the machine is near neutral, has low displacement, or is inching.
Various tiltable swashplate arrangements are known for varying displacement in axial piston pumps and motors. In one arrangement, the swashplate has opposite cylindrical trunnions that pivotally mount or journal it in the pump or motor housing. A plurality of pistons slidably mount in corresponding piston bores or chambers arranged in a circular pattern in a rotatable cylinder block that is urged by a block spring toward the tiltable swashplate. A valve plate engages the end of the cylinder block that is remote from the swashplate. Slippers swivelingly attached to the pistons engage a running surface on the swashplate as the cylinder block rotates. If the running surface of the swashplate is perpendicular to the longitudinal axes of the pistons, the pistons do not reciprocate in the cylinder block and no fluid is displaced or consumed by the hydraulic unit. A lubrication hole typically extends longitudinally through the piston and slipper so that oil from the piston bore or chamber can reach the slipper running surface of the swashplate.
When the swashplate is forcibly tilted away from perpendicular, the pistons reciprocate in the piston bores as the pistons are driven in a circle against the inclined plane. This reciprocating action means that the chambers of the pistons on one region of the swashplate are under high pressure, while the piston chambers on the opposite region of the swashplate are under low pressure. Each piston bore or chamber in the cylinder block has a xe2x80x9cpressure profilexe2x80x9d associated with it as the block rotates. The pressure acting on the cross-sectional area of the piston translates into a force, which yields a moment on the swashplate. To move or maintain the swashplate tilted to given degree, a moment of equal and opposite magnitude must be maintained on the swashplate. The operator does this manually by applying a force on a lever or torque on a handle attached to the swashplate or through a conventional servo mechanism. If a servo mechanism is used, operator xe2x80x9cfeelxe2x80x9d is usually lost.
One common method of fine tuning or affecting swashplate moments in a hydrostatic unit is a static method involving designing a specific valve plate with a specific fixed porting configuration to achieve the desired swashplate moments. A valve plate is a substantially flat disc-shaped annular ring of material that is fixed against rotation on the end cap of the hydraulic unit adjacent the rear surface of the rotating cylinder block (which is opposite of the swashplate). The conventional valve plate typically has an arcuate inlet port and an arcuate outlet port formed therethrough on opposing sides of a median axis. These ports reside along arcs that generally align with the pitch circle of the piston bores in the cylinder block. Thus, the inlet and outlet ports generally register with the circular path of the reciprocating pistons as the pistons rotate with the cylinder block against the valve plate. The inlet and outlet ports are angularly spaced apart in the areas or zones where the reciprocating pistons change their direction of reciprocal movement or transition from high pressure to low pressure and vice versa. The top dead center (TDC) and bottom dead center (BDC) positions of the reciprocating pistons generally correspond to these transition zones. The spacing of the inlet and outlet ports of the valve plate depends to some extent on the number of pistons in the rotating cylinder block assembly.
Some existing valve plates utilize specially shaped notches, such as xe2x80x9crat tailsxe2x80x9d or xe2x80x9cfish tails,xe2x80x9d at the entrance and/or exit of the ports (i.e.xe2x80x94in the transition zones) to affect the swashplate moments. Moon et al. U.S. Pat. No. 3,585,901 teaches the basics of utilizing valve plate fish tails to affect swashplate moments in axial piston hydraulic units. U.S. Pat. No. 4,550,645 teaches some additional geometric configurations for fish tails and valve plates. Unfortunately, many different valve plates are required to satisfy the swashplate moment demands of the various users. Thus, the number of valve plate designs tends to proliferate and it can be costly to produce and warehouse an adequate selection of valve plates. Furthermore, if a change in swashplate moments is desired, the user must physically disassemble the unit and change the valve plate. Finally, the valve plate configuration is essentially constant or static once a particular valve plate is selected and installed. A valve plate configuration may have beneficial effects on the swashplate moments, performance and controllability of the unit under certain operating conditions (including but not limited to speed, pressure and displacement), but the same valve plate configuration may have undesirable effect under other conditions within the normal operating range of the unit. Since the valve plate geometry is fixed based upon the valve plate chosen, the user must accept the tradeoffs involved. Careful and elaborate optimization analysis is often required to determine the best valve plate design for the task.
Thus, there is a need for dynamic rather than static means and methods for affecting swashplate moments. There is also a need for a means and method for affecting swashplate moments that does not necessarily involve valve plate design changes or valve plate proliferation.
Crawlers are large machines that utilize servo systems to control the position of the swashplate. The size of the servo systems can become quite bulky or require high control pressure, limiting the response of the swashplate. Thus, there is a need for a means for reducing the power requirements of the servo system, allowing smaller servo systems and/or lower control pressures.
In the mobile hydraulic market increasing demands are being made for lower noise, lower flow ripple and higher efficiency. In the past, one selected from among a variety of valve plates having fixed porting designs to control the power level requirements for positioning the swashplate. Sacrifices in noise, flow ripple and efficiency were made to achieve the desired power requirements. Thus, there is a need for a means for controlling the power level requirements while optimizing noise, flow ripple and efficiency.
Therefore, a primary objective of the present invention is the provision of a dynamic means and method for affecting the cylinder block piston bore pressure profile in a hydrostatic unit.
Another objective of this invention is the provision of a variable means of affecting swashplate moments throughout the normal operating range of operating conditions of the hydraulic unit.
Another objective of this invention is the provision of a means for reducing net swashplate moments in a manually controlled hydraulic unit to reduce operator fatigue without sacrificing the feel of operator feedback.
Another objective of this invention is the provision of a means for generating a control error signal to a variable orifice valve for bleeding fluid between adjacent pistons to affect bore pressure and subsequently swashplate moments.
Another objective of this invention is the provision of means for varying swashplate moments without the need for changing valve plates in a hydraulic unit.
Another objective of this invention is the provision of a method for optimizing piston bore pressures that allows the operator to feel connected to the machine while reducing the power level requirement from the operator.
Another objective of this invention is the provision of a method of reducing power requirements that is also applicable to servo-controlled units so as to allow smaller servo systems and/or control pressures.
Another objective of this invention is the provision of a means for controlling the power level requirements while optimizing noise, flow ripple and efficiency.
Another objective of this invention is the provision of a means for controlling a system variable, including but not limited to pressure ripple, flow ripple, noise, vibration, efficiency, and/or control force or power requirements. An example is a means for reducing the noise level in a hydraulic unit at all operating conditions regardless of moment levels.
These and other objectives will be apparent from the drawings, as well as from the description and claims that follow.
The present invention relates to an electronic bore pressure optimization mechanism for dynamically varying swashplate moments in a multiple piston hydraulic unit. The mechanism includes a variable orifice associated with a bleed passage in the end cap or center section of the hydraulic unit. The fluid passage comes into communication with the block kidneys of individual piston bores as the piston bores move along the pitch circle and through the transition area during rotation of the cylinder block. The variable orifice effectively resides between a first piston or pressure source and an adjacent transitioning pumping/motoring piston. The mechanism utilizes one or more sensed parameters from a group including but not limited to noise, pressure, speed, swashplate position, swashplate control requirements, vibration, and operator input to electronically control the variable orifice and thereby meter the flow of fluid to and from the transitioning pistons. The mechanism can be associated with the low pressure source side of the loop or the high pressure source side of the closed circuit loop. Optionally, a valve plate can be positioned between the end cap and the cylinder block and provided with a non-limiting fluid passage that connects the block kidney and the bleed passage in the end cap.
The invention adapts equally well to manually controlled units and servo-assisted units.